Perhaps this is caused by the purging of images 6 months and older?
Step 5/12 : RUN apk update && apk upgrade && apk add $BUILD_PACKAGES && apk add $RUBY_PACKAGES && rm -rf /var/cache/apk/*
---> Running in b49faf65af04
fetch http://dl-4.alpinelinux.org/alpine/v2.7/main/x86_64/APKINDEX.tar.gz
wget: server returned error: HTTP/1.1 404 Not Found
Perhaps this is caused by the purging of images 6 months and older?